The analysis of samples by mass spectrometry requires the analyte material to be ionized such that the mass-to-charge ratios (m/z's) of the resultant gas-phase ions can be determined. Ionization techniques have been developed over the years to charge sample atoms and molecules based on their inherent polarity, stability and size. Positively-charged ions (cations) or negatively-charged ions (anions) generated by ionization of the sample in an ion source are then transferred to downstream regions of a mass spectrometer via an inlet for subsequent processing (e.g., dissociation) and separation according to their m/z's. The inlet is provided with a passageway, such as the central bore of an ion transfer tube or a sampler cone orifice, permitting the transfer of ions and entraining gas therethrough. Typically, transfer of ions to the downstream regions through the inlet occurs under the influence of electrostatic field and/or pressure gradients.
During operation of the mass spectrometer, some fraction of the material emitted by the ion source impinges upon inlet surfaces adjacent to the passageway. Accumulation of this material on the inlet surfaces, which may be particularly prevalent when certain types of ionization sources are utilized and/or when “dirty” sample matrices (e.g., whole blood or urine) are analyzed, may eventually interfere with the passage of ions through the inlet, either by physically blocking a portion of the passageway or by altering electrical fields, leading to reduced sensitivity. Furthermore, material deposited on inlet surfaces during analysis of one sample may contaminate subsequently analyzed samples, producing sample crossover and yielding erroneous results.
In order to clean the residue from inlet surfaces and avoid the problems associated with excessive accumulation, it is typically necessary for the instrument operator to manually clean the affected surface, for example by applying a suitable solvent and wiping the surface with a cleaning cloth. This procedure requires that operation of the instrument be interrupted, and may also require the operator to disassemble some portion of the ion source to allow access to the inlet surface, both of which may be undesirable.
U.S. Pat. No. 6,380,538 discloses an ion source having a conduit that continuously dispenses cleaning fluid, such as water, onto a surface of an orifice member that forms the inlet into lower pressure regions of a mass spectrometer. The dispensed fluid then flows toward and over the orifice edge and into the lower pressure regions under the influence of the pressure gradient. The action of continuously dispensing cleaning fluid onto the orifice member surface is said to reduce or avoid the problem of orifice blocking arising from the deposition of involatile substances near the orifice. While this approach may be used with some success for certain instruments and applications, the continuous flow of cleaning fluid into the mass spectrometer may interfere with the measurement of analytes of interest. Furthermore, the action of the continuously flowing cleaning fluid may cause involatile substances to be passed to the lower pressure regions of the mass spectrometer, where they may contaminate surfaces of ion optics or mass analyzers and adversely impact instrument performance.
What is needed is a method and apparatus for cleaning the inlet of the mass spectrometer without interruption to instrument operation or removal of the ion source, and which prevents the introduction of undesirable materials into the mass spectrometer.